A non-aqueous electrolyte secondary battery, which is typified by a lithium ion secondary battery, is widely used as a main electric power source of portable electronic equipment such as cellular phones and a notebook computers. Further, a non-aqueous electrolyte secondary battery is broadly used as a main electric power source of electric cars or hybrid cars, power storage systems for night time electricity, or the like. As a result of the wide spread use of non-aqueous electrolyte secondary batteries, ensuring stable battery characteristics and safety are important issues.
A separator plays an important role in ensuring the safety of a nonaqueous electrolyte secondary battery. As it now stands, a porous film containing polyolefin as a main component is used from the viewpoint of, in particular, a shutdown function. In the field of non-aqueous electrolyte secondary batteries, a shutdown function means the function of blocking an ion flow by closing the pores of the porous film as a result of melting polyolefin when abnormal heat generation occurs in the battery, thereby preventing the battery from suffering thermal runaway.
However, when the battery temperature further increases even after the shutdown function starts, the entire polyolefin porous film melts (a so-called “melt-down”). As a result, a short circuit occurs inside the battery and, as a large amount of heat is generated in conjunction with the short circuit, and there is a risk of smoke generation, ignition or an explosion occurring in the battery. For such reasons, a separator is required to have not only a shutdown function, but also heat resistance that does not allow a melt-down even at a temperature higher than the temperature at which the shutdown function commences.
In order to prevent a melt-down of a separator, it has been proposed to form a heat resistant layer containing a heat resistant resin, such as polyamide, on one or both surfaces of a polyolefin porous film (e.g., see Patent Documents 1 to 12). According to these proposals, the ensuring of safety of a battery at high temperature can be expected since both the shutdown function and the inhibition of a melt-down are achieved.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2002-355938    Patent Document 2: JP-A No. 2005-209570    Patent Document 3: JP-A No. 2005-285385    Patent Document 4: JP-A No. 2000-030686    Patent Document 5: JP-A No. 2009-205959    Patent Document 6: JP-A No. 2009-143060    Patent Document 7: JP-A No. 2008-080536    Patent Document 8: WO 2007/013179 A    Patent Document 9: WO 2008/156033 A    Patent Document 10: WO 2008/062727 A    Patent Document 11: WO 2008/149895 A    Patent Document 12: WO 2010/021248 A